1. Field of the Invention
The present invention relates to an optical communication system, and more specifically, it relates to a chromatic dispersion compensation technology as an indispensable technology for realizing an ever-progressing large-capacity, high-speed and long-haul optical communication system. In particular, it relates to an automatic dispersion compensation device optimally compensating for chromatic dispersion and polarization-mode dispersion in a transmission line and a compensation method thereof.
2. Description of the Related Art
Recently, although the network capacity has increased rapidly, demand for an even larger capacity of the network has also been grown. Although currently a wavelength division multiplexing (WDM) optical communication method based on a transfer rate of 10 Gb/s per channel is already put into practical use, a far larger capacity is needed in the future. In these situations, the improvement of frequency usage efficiency and the realization of an ultra-high-speed optical communication system with a transfer rate of 40 Gb/s per channel or more are expected from the viewpoint of equipment cost and size.
However, since in such an ultra-high-speed optical communication system, the influence on the transmission quality, that is, transmission waveform degradation due to chromatic dispersion and polarization-mode dispersion, increases, thus limiting the transmission distance of optical signals, which is a problem. For this reason, a highly accurate compensation method is needed for chromatic dispersion and polarization-mode dispersion to realize such an ultra-high-speed optical communication system. Both chromatic dispersion and polarization-mode dispersion are described below.
(1) Chromatic Dispersion
In an optical communication system with a transfer rate of more than 10 Gb/s, a tolerance for chromatic dispersion is remarkably small. For example, the chromatic dispersion tolerance of a 40 Gb/s non-return-to-zero (NRZ) system is 100 ps/nm (pico-sec/nm) or less.
Generally, the repeater spacing of an optical communication system is not constant. For this reason, if, for example, 1.3 μm zero-dispersion single mode fiber (SMF) with a chromatic dispersion value of 17 ps/nm/km is used, chromatic dispersion deviates from its tolerance threshold when repeaters are apart by only several kilo-meters.
On the other hand, since the distance between repeaters and the chromatic dispersion value of an optical fiber transmission line possessed by a communication carrier are not accurately known, it is often difficult to realize highly accurate chromatic dispersion compensation using a fixed-chromatic dispersion compensation method adopting a dispersion compensated fiber (DCF) and the like.
Furthermore, since a chromatic dispersion value varies depending on fiber temperature, stress and the like, as time elapses, the amount of chromatic dispersion of each span must be optimally adjusted by strictly measuring chromatic dispersion not only at the time of the start of the system operation but also during the system operation. For example, if the type of an optical fiber, the length of a transmission line and temperature fluctuation are DSF, 500 km and 100° C., respectively, the following equation holds true.[Amount of wavelength dispersion]=[Temperature dependence of zero-dispersion wavelength]×[Amount of temperature change in transmission line]×[Dispersion slope of transmission line (Dispersion slope is the chromatic dependence of chromatic dispersion)]×[Transmission distance]=0.03 nm/° C.×100° C.×0.07 ps/nm2/k×500 km=105 ps/nmThis value is almost equivalent to the chromatic dispersion tolerance threshold of a 40 Gb/s NRZ signal. Therefore, an automatic wavelength compensation system, always monitoring the chromatic dispersion value in a transmission line and optimally controlling the amount of wavelength compensation, is indispensable not only in an SMF transmission line but also in a system using a 1.55 μm zero-dispersion shift fiber (DSF) or an NZ (non-zero)-DSF for a transmission line.(2) Polarization-Mode Dispersion
Next, polarization mode dispersion (PMD) is described.
PMD is dispersion due to the respective different propagation delay times of the polarization elements (two pieces of mode light: for example, a TE mode light and TM mode light) of an optical signal, and it can occur in all types of optical fibers.
Generally, the larger the amount of an optical signal is or the longer the transmission distance of an optical signal is, the greater the influence of polarization mode dispersion becomes, which cannot be neglected. It is said that an optical fiber constituting an old optical transmission line, mainly laid in countries other than Japan, has a large PMD value exceeding 1 ps/km1/2 (pico-sec/km1/2: one pico is 10−12) per unit length. Even when short-haul transmission (for example, 50 km transmission) is conducted using such an optical fiber, an optical delay difference (Δτ) for one time slot of 25 ps of a 40 Gb/s NRZ signal is 7 ps or more. Therefore, the influence of polarization-mode dispersion is also not negligible as in the case of chromatic dispersion described above. In reality, since components causing polarization-mode dispersion, such as an optical amplifier, a chromatic dispersion compensator and the like, must be installed in the transmission line of an optical communication system, there is a possibility that the transmission distance of an optical signal will be further limited. Furthermore, since polarization-mode dispersion varies depending on stress or temperature change that are placed on an optical fiber as time elapses, the state of polarization-mode dispersion must be monitored and be dynamically compensated for not only at the time of the construction of a system but also during the operation.
As described above, chromatic dispersion and polarization-mode dispersion are major factors for limiting the performance of an optical communication system. In order to improve the performance of an optical communication system, an automatic dispersion compensation system individually and dynamically compensating for both chromatic dispersion and polarization-mode dispersion must be provided.
Three element technologies needed to realize such an automatic dispersion compensator are as follows:    (a) Realization of a variable chromatic dispersion compensator    (b) Realization of chromatic dispersion monitoring in a transmission line    (c) Realization of a feedback optimal control method for a variable chromatic dispersion compensator (However, at the following (c) is not described.)
As for the chromatic dispersion compensator of (a), for example, the following items are proposed:
(1) VIPA (Virtually Imaged Phased Array)
“Variable Dispersion Compensator Using the Virtually Imaged Phased Array (VIPA) for a 40 Gbit/s WDM Transmission System”, ECOC2000, PD Topics 2, 2.3.
(2) Tunable Ring Resonator
“Tunable Ring Resonator Dispersion Compensators realized in High Refractive-index Contrast Technology”, ECOC2000, PD Topic 2, 2.2.
(3) FBG (Fiber Bragg Grating)
“Twin Fiber Grating Adjustable Dispersion Compensator for 40 Bbit/s”, ECOC2000, PD Topic 2, 2.4.
As for the polarization-mode dispersion compensator, for example, the following items are proposed:    (1) A method for controlling a polarization controller (PC), in such away that the optical intensity divergence ratio γ of two polarization modes is 0 or 1, by providing an optical signal transmitting terminal with the PC and feeding back a transmission characteristic from the receiving terminal,
“Optical Equalization of Polarization Dispersion”, SPIE Vol. 1, 1787, Multigigabit fiber Communications (1992), pp. 346-357.    (2) A method for causing a delay difference between two polarization modes, with a sign that is the reverse of that of an optical transmission line, by providing an optical signal receiving terminal with a polarization controller and a polarization maintaining fiber (PMF), and controlling the polarization controller,
“Automatic Compensation Technique for Timewise Fluctuating Polarization Mode Dispersion in In-line Amplifier Systems”, Electro, Lett., Vol. 30, No. 4, 994, pp. 348-349.    (3) A method for controlling a polarization controller and a variable delay element by providing a polarization controller and a polarization beam splitter (PBS), wherein two optical receivers each receiving one of two optical signal elements, split by this polarization beam splitter, and a variable delay element which causes a delay difference between two electrical signals obtained from these optical receivers,
“Polarization Control Method for Suppressing Polarization Mode Dispersion Influence in Optical Transmission Systems”, J. of Lightwave Technol., Vol. 12, No. 5 (1994), pp. 891-898.
Next, several methods are also proposed for chromatic dispersion monitoring in a transmission line (b) that is indispensable for feedback control.
Firstly, as a method for measuring chromatic dispersion values, a pulse method for inputting a plurality of rays each with a different wavelength to an optical fiber, and a method measuring the group delay between output rays or the phase difference have been conventionally proposed. However, in order to be able always to measure chromatic dispersion without communication quality degradation during the system operation, (1) one set of chromatic dispersion measuring instruments is needed for each repeater section and (2) the measurement of light with a wavelength different from that of the data signal must be wavelength-multiplexed, both of which are problems. It is not practical from the viewpoints of economical efficiency and device size to realize such measures.
As examples of such achromatic dispersion monitor for solving these problems, several methods are proposed. Examples of such a chromatic dispersion monitor are described below.    (1) A method using the intensity of a specific frequency element of an incoming base-band signal utilizing the property that the intensity of a specific frequency element changes due to waveform distortion, (“Automatic Dispersion Equalization in 40 Gbit/s Transmission by Seamless-switching between Multiple Signal Wavelengths”, ECOC'99, pp. I-150-151.    (2) A method using an error rate
A method for monitoring an error rate using a receiver and feedback which controls a chromatic dispersion compensator in such a way that the error rate becomes optimized (“Optical Fiber Communication System Incorporating an Automatic Dispersion Compensation Module Compensating for the Fluctuations of Dispersion Due to Temperature”, Japanese Patent Laid-open No. 2001-77756 (P2001-77756A) and “Automatic Equalization System”, Japanese Patent Laid-open No. 9-326755)
As a method for measuring polarization-mode dispersion, the following items are proposed:    (1) Modulated Phase-Shift Method    (2) Jones Matrix Eigen Analysis method    (3) Poincare' Sphere Analysis method    (4) Interferometric method
As a method for displaying (expressing) a polarization state, the following are proposed (“Method for Displaying and Measuring Polarization State”, Optronics (1997), No. 5, pp. 109-117):    (1) Poincare sphere    (2) Jones' vector    (3) Stokes' vector
A method for measuring polarization-mode dispersion using Jones' vector and a device thereof are proposed in Japanese Patent Laid-open No. 9-72827 as one example. A polarization dispersion monitor monitoring a specific frequency element of an incoming signal is also proposed although it is difficult to apply it in an environment where there is chromatic dispersion.
Any practical chromatic dispersion monitor directly or indirectly uses waveform distortion due to dispersion. In this case, if there are simultaneously chromatic dispersion and polarization-mode dispersion, waveform distortion due to these two forms of dispersion cannot be distinguished. Therefore, it is difficult to realize an automatic dispersion compensator simultaneously compensating for both chromatic dispersion and polarization-mode dispersion.
Furthermore, if an already-proposed parameter indicating transmission quality, such as an error rate and the like, is used instead of a chromatic dispersion monitor, it is difficult to separate the degradation of transmission quality due to chromatic dispersion from that due to factors other than it. Therefore, in this case, factors for transmission quality degradation are not separated and control is exercised as if a chromatic dispersion compensator could compensate for transmission quality degradation due to all factors. However, since there are various factors of transmission quality degradation, having only a chromatic dispersion compensator means that it cannot always compensate for all transmission quality degradation. Therefore, there is no guarantee that optimal control can be always exercised, and furthermore sometimes there will be no control.
If in this way, a chromatic dispersion compensator is operated without separating the factors of transmission quality degradation, optimal control cannot be guaranteed and sometimes there is no control. Such a case is described below with reference to FIGS. 1 through 3.
FIG. 1 shows the entire configuration of a conventional optical communication system used to describe the problems. In FIG. 1, a multiplexer 101 multiplexes the outputs of a transmitter TX 100, for each channel (100 GHz interval between channels) from channel 1 (196 THz) to channel 40 (192.1 THz), and transmits it to a receiver through a transmission line of, for example, 90 km. It is assumed that dispersion in the transmission line is 5.0 ps/nm/km for channel 1 and the dispersion slope is 0.06 ps/nm2/km.
On the receiver side, a demultiplexer 103 demultiplexes signals for each channel, and a variable chromatic dispersion compensator (VDC) 104 compensates for the chromatic dispersion of signals in each channel. Then, the VDC 104 transmits these signals to a receiver RX 105. Then, a monitor/controller 106 monitors the transmission quality of the received result and controls the VDC 104. In the following description, the non-linear effect of a fiber is neglected for conveniences' sake and the number of errors experienced is used as the amount of monitoring of the monitor/controller 106.
FIG. 2 shows the relation between the average number of errors per second and the residual amount of chromatic dispersion obtained when the decision threshold and decision phase of the receiver is optimized. If the allowable number of, penalties of error due to chromatic dispersion is 1, dispersion tolerance is approximately 98 ps/nm.
The case where the deviation of the decision threshold of the receiver is one factor for transmission quality degradation, other than chromatic dispersion, that is studied. FIG. 3 shows the relation between the number of errors and the residual amount of chromatic dispersion of the decision threshold in which there are errors with the allowable number of errors equal to 1.
Compared with FIG. 2, in FIG. 3, there is no control and it is difficult to detect an optimal chromatic dispersion value. In other words, in such a situation, the control of only a dispersion compensator cannot realize a transmission that matches the allowable number of errors.
If there is transmission quality degradation due to factors other than chromatic dispersion, no operation of a chromatic dispersion compensator is actually needed. Therefore, in order to control a chromatic dispersion compensator using a transmission quality monitor, the factors causing transmission quality degradation must be separated, which is a problem.